The number of persons with Alzheimer’s disease is increasing world-wide and the disease affects the persons, their families, the health care system and the economy within society worldwide. The symptoms and behaviours caused by Alzheimer’ disease may be difficult to manage for the person and their caregivers. Alternative methods are recommended before pharmacological treatment. The presence of a therapy dog has been described as beneficial, in for instance increasing well-being and alleviating symptoms and dementia behaviours. The overall aim of this thesis was to gain a deeper understanding of the influence of therapy dogs on persons with Alzheimer’s disease from the person’s and the dog handler’s perspectives. Further, adopting a longitudinal perspective, the study investigates the therapy dog’s influence on activity and sleep for persons with Alzheimer’s disease. Video observations of five persons with Alzheimer’s disease interacting with a therapy dog (I, II), as well as interviews with nine dog handlers (III) were gathered and transcribed. Data was analyzed using a phenomenological hermeneutical method (I, II, III). Registration of activity and sleep was conducted over a period of 16 weeks using an Actigraf that generated curves, and were then analysed using descriptive statistics (III). The time spent with the dog revealed memories and feelings resulting in existential thoughts of oneself and life, which then connected to the present situation (I). Distancing oneself from the symptoms of the disease when interacting with the dog showed a person functioning in the present with the dog, striving for the dog’s best and putting the dog before and above oneself (II). The therapy dog’s presence showed no pattern of effect on the patients’ daytime activity and sleep. The findings instead pointed to a great variety of possible different effects, bringing about increased activity at different time points, for example during night-time sleep (III), creating a respite from illness and contributing wordlessly to an existence but thoroughly directed by the dog handler, where the person was comfortable and took the initiative (IV). In conclusion, the therapy dog team’s presence with the person with Alzheimer’s disease induced meaning that allowed the person’s hidden qualities and abilities to develop and, when observed from a person-centred perspective, also brought out the individual in each person